Post Weld Heat Treatment (PWHT),defined as any heat treatment after welding, is often used to improve the properties of a welding. In concept, PWHT can encompass many different potential treatments; however, in steel fabrication, the two most common procedures used are post heating and stress relieving

PWHT  in the context described here refers to the process of reheating a weld to below the lower transformation temperature at a controlled rate, holding for a specific time and cooling at a controlled rate. No consideration has been given to normalizing of welds in carbon-manganese and low alloy steels, to solution annealing of stainless steels or to any precipitation hardening treatments to other alloy materials

Advantages to Improving the diffusion of hydrogen out of weld metal. Softening the heat affected zone and thus improving toughness. Improving dimensional reliability during machining. Improving ductility. Improving the resistance to stress corrosion cracking. Ease the effects of cold work.